<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>无标题文档</title>
<style>
#one{width:100px;height:200px;background:red;position:absolute;top:0;right:0;}
</style>
<script>
window.onscroll=function(){
	var one=document.getElementById('one');
	var scrTop=document.documentElement.scrollTop||document.body.scrollTop;
	//one.style.top=scrTop+(document.documentElement.clientHeight-one.offsetHeight)/2+'px';
	var t=scrTop+(document.documentElement.clientHeight-one.offsetHeight)/2;
	startMove(parseInt(t))
	
};

var timer=null
function startMove(target){
	var one=document.getElementById('one');
	clearInterval(timer);
	timer=setInterval(function(){
		var speed=(target-one.offsetTop)/8;
		speed=speed>0?Math.ceil(speed):Math.floor(speed);
		if(one.offsetTop==target){
			clearInterval(timer);	
		}else{
			one.style.top=one.offsetTop+speed+'px';	
			
			txt.value=one.offsetTop+',目标：'+target;
		};
		
		
	},30)	
};
</script>
</head>

<body style="height:2000px">
<div id="one"></div>
<input type="text" id="txt" style="position:fixed; top:50px;left:200px;" />
</body>
</html>
